Abstract: A VAV box includes a housing formed by a top wall, a bottom wall, and a pair of vertical side walls. A blade is mounted for rotation about a vertical axis within an air flow passageway defined by a hollow interior of the housing. A first protuberance formed on an interior side of the first side wall has a leading wall swept back relative to an entrance to the box, a trailing wall swept forward relative to the entrance, and a concave wall that interconnects the innermost ends of the leading and trailing walls. A second protuberance having the same structure is formed on an interior side of the second side wall. The protuberances produce a linear response in air flow to changes in blade position, eliminating the non-linear response of prior art VAV boxes.